1. Field of the Invention
Embodiments of the invention generally relate to systems for immersing a substrate in an electrolyte solution, and more particularly, to apparatuses and methods for controlling electrical power applied to a substrate during an immersion process.
2. Description of the Related Art
Metallization of sub-quarter micron sized features is a foundational technology for present and future generations of integrated circuit manufacturing processes. More particularly, in devices such as ultra large scale integration-type devices, i.e., devices having integrated circuits with more than a million logic gates, the multilevel interconnects that lie at the heart of these devices are generally formed by filling high aspect ratio interconnect features with a conductive material, such as copper or aluminum, for example. Conventionally, deposition techniques such as chemical vapor deposition (CVD) and physical vapor deposition (PVD), for example, have been used to fill these interconnect features. However, as interconnect sizes decrease and aspect ratios increase, void-free interconnect feature fill via conventional metallization techniques becomes increasingly difficult. As a result thereof, plating techniques, such as electrochemical plating (ECP) and electroless plating, for example, have emerged as viable processes for void free filling of sub-quarter micron sized high aspect ratio interconnect features in integrated circuit manufacturing processes.
In an ECP process, for example, sub-quarter micron sized high aspect ratio features formed into the surface of a substrate may be efficiently filled with a conductive material, such as copper, for example. ECP plating processes are generally two stage processes, wherein a seed layer is first formed over the surface features of the substrate, and then the surface features of the substrate are exposed to an electrolyte solution, while an electrical bias is simultaneously applied between the substrate and an anode positioned within the electrolyte solution. The electrolyte solution is generally rich in ions to be plated onto the surface of the substrate, and therefore, the application of the electrical bias causes these ions to be urged out of the electrolyte solution and to be plated onto the seed layer.
However, one challenge associated with ECP processes is that the plating solutions, which are generally acidic, may chemically etch the seed layer during the process of immersing the substrate in the plating solution. Inasmuch as seed layers are generally relatively thin conductive layers, the chemical etching that may take place during the immersion process may result in conductive discontinuities in the seed layer, which may cause plating irregularities in the subsequent plating processes. Therefore, in order to prevent chemical etching during the immersion process, conventional ECP plating systems apply an electrical loading bias to the substrate seed layer during the immersion process. The loading bias is generally configured to apply a cathodic bias to the substrate in order to prevent the chemical etching process and to cause at least some forward plating to occur. A conventional loading bias generally includes a constant voltage or current being applied to the substrate as the substrate is immersed in the plating solution in a tilt-type motion, i.e., one edge of the substrate is first immersed, and then a tilting motion is used to immerse the remaining portion of the substrate. This generally operates to urge bubbles that may form on the surface of the substrate toward the upper end of the substrate being tilted into the plating solution, and therefore, remove the bubbles from the surface, so that plating irregularities that may result from bubbles are minimized. However, as the surface of the substrate is immersed in the tilting motion under the constant load bias, the conductive area of the substrate being immersed is changing (increasing), and therefore, the resistance of the conductive path is also changing (decreasing). As a result of the changes in the current path during the immersion process, the current density applied across the surface of the substrate varies through the immersion process. Although the immersion process may only take a few seconds, the uniformity of the thin conductive seed layer may be affected by the varying current density applied across the surface of the seed layer, and therefore, the uniformity of layers subsequently plated over the seed layer may also be affected.
Therefore, there is a need for an apparatus and method for applying a loading bias to a substrate being immersed in a plating solution, wherein the method and apparatus provide a uniform current density across the surface of the substrate throughout the immersion process.